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.TH "BTRFS-SCRUB" "8" "Nov 29, 2024" "6.12" "BTRFS"
.SH NAME
btrfs-scrub \- scrub btrfs filesystem, verify block checksums
.SH SYNOPSIS
.sp
\fBbtrfs scrub\fP <subcommand> <args>
.SH DESCRIPTION
.sp
Scrub is a validation pass over all filesystem data and metadata that detects
data checksum errors, basic super block errors, basic metadata block header errors,
and disk read errors.
.sp
Scrub is done on a per\-device base, if a device is specified to \fBbtrfs scrub start\fP,
then only that device will be scrubbed. Although btrfs will also try to read
other device to find a good copy, if the mirror on that specified device failed
to be read or pass verification.
.sp
If a path of btrfs is specified to \fBbtrfs scrub start\fP, btrfs will scrub
all devices in parallel.
.sp
On filesystems that use replicated block group profiles (e.g. RAID1), read\-write
scrub will also automatically repair any damage by copying verified good data
from one of the other replicas.
.sp
Such automatic repair is also carried out when reading metadata or data from a
read\-write mounted filesystem.
.sp
\fBWARNING:\fP
.INDENT 0.0
.INDENT 3.5
As currently implemented, setting the \fBNOCOW\fP file attribute (by
\fBchattr +C\fP) on a file implicitly enables
\fBNODATASUM\fP\&. This means that while metadata for these files continues to
be validated and corrected by scrub, the actual file data is not.
.sp
Furthermore, btrfs does not currently mark missing or failed disks as
unreliable, so will continue to load\-balance reads to potentially damaged
replicas. This is not a problem normally because damage is detected by
checksum validation, but because \fBNOCOW\fP files are
not protected by checksums, btrfs has no idea which mirror is good thus it can
return the bad contents to the user space tool.
.sp
Detecting and recovering from such failure requires manual intervention.
.sp
Notably, \X'tty: link https://github.com/systemd/systemd/commit/11689d2a021d95a8447d938180e0962cd9439763'\fI\%systemd sets +C on journals by default\fP\X'tty: link',
and \X'tty: link https://www.libvirt.org/news.html#v6-6-0-2020-08-02'\fI\%libvirt ≥ 6.6 sets +C on storage pool directories by default\fP\X'tty: link'\&.
Other applications or distributions may also set \fB+C\fP to try to improve
performance.
.UNINDENT
.UNINDENT
.sp
\fBNOTE:\fP
.INDENT 0.0
.INDENT 3.5
Scrub is not a filesystem checker (fsck, \fI\%btrfs\-check(8)\fP). It can only detect
filesystem damage using the checksum validation, and it can only repair
filesystem damage by copying from other known good replicas.
.sp
\fI\%btrfs\-check(8)\fP performs more exhaustive checking and can sometimes be
used, with expert guidance, to rebuild certain corrupted filesystem structures
in the absence of any good replica.
.UNINDENT
.UNINDENT
.sp
The user is supposed to run it manually or via a periodic system service. The
recommended period is a month but it could be less. The estimated device bandwidth
utilization is about 80% on an idle filesystem.
.sp
The scrubbing status is recorded in \fB/var/lib/btrfs/\fP in textual files named
\fIscrub.status.UUID\fP for a filesystem identified by the given UUID. (Progress
state is communicated through a named pipe in file \fIscrub.progress.UUID\fP in the
same directory.) The status file is updated every 5 seconds. A resumed scrub
will continue from the last saved position.
.sp
Scrub can be started only on a mounted filesystem, though it\(aqs possible to
scrub only a selected device. See \fI\%btrfs scrub start\fP for more.
.SS Bandwidth and IO limiting
.sp
\fBNOTE:\fP
.INDENT 0.0
.INDENT 3.5
The \X'tty: link https://man7.org/linux/man-pages/man1/ionice.1.html'\fI\%ionice(1)\fP\X'tty: link' may not be generally supported by all IO schedulers and
the options to \fBbtrfs scrub start\fP may not work as expected.
.UNINDENT
.UNINDENT
.sp
In the past when the \X'tty: link https://en.wikipedia.org/wiki/Completely_fair_queueing'\fI\%CFQ IO scheduler\fP\X'tty: link' was generally used
the \X'tty: link https://man7.org/linux/man-pages/man1/ionice.1.html'\fI\%ionice(1)\fP\X'tty: link' syscalls set the priority to \fIidle\fP so the IO would not
interfere with regular IO. Since the kernel 5.0 the CFQ is not available.
.sp
The IO scheduler known to support that is \X'tty: link https://docs.kernel.org/block/bfq-iosched.html'\fI\%BFQ\fP\X'tty: link', but first read the
documentation before using it!
.sp
For other commonly used schedulers like \X'tty: link https://docs.kernel.org/block/blk-mq.html'\fI\%mq\-deadline\fP\X'tty: link' it\(aqs recommended to use
\fIcgroup2 IO controller\fP which could be managed by e.g. \fIsystemd\fP
(documented in \fBsystemd.resource\-control\fP). However, starting scrub like that
is not yet completely straightforward. The IO controller must know the physical
device of the filesystem and create a slice so all processes started from that
belong to the same accounting group.
.INDENT 0.0
.INDENT 3.5
.sp
.EX
$ systemd\-run \-p \(dqIOReadBandwidthMax=/dev/sdx 10M\(dq btrfs scrub start \-B /
.EE
.UNINDENT
.UNINDENT
.sp
Since linux 5.14 it\(aqs possible to set the per\-device bandwidth limits in a
BTRFS\-specific way using files \fB/sys/fs/btrfs/FSID/devinfo/DEVID/scrub_speed_max\fP\&.
This setting is not persistent, lasts until the filesystem is unmounted.
Currently set limits can be displayed by command \fI\%btrfs scrub
limit\fP\&.
.INDENT 0.0
.INDENT 3.5
.sp
.EX
$ echo 100m > /sys/fs/btrfs/9b5fd16e\-1b64\-4f9b\-904a\-74e74c0bbadc/devinfo/1/scrub_speed_max
$ btrfs scrub limit /
UUID: 9b5fd16e\-1b64\-4f9b\-904a\-74e74c0bbadc
Id      Limit      Path
\-\-  \-\-\-\-\-\-\-\-\-  \-\-\-\-\-\-\-\-
 1  100.00MiB  /dev/sdx
.EE
.UNINDENT
.UNINDENT
.SH SUBCOMMAND
.INDENT 0.0
.TP
.B cancel <path>|<device>
If a scrub is running on the filesystem identified by \fIpath\fP or
\fIdevice\fP, cancel it.
.sp
If a \fIdevice\fP is specified, the corresponding filesystem is found and
\fBbtrfs scrub cancel\fP behaves as if it was called on that filesystem.
The progress is saved in the status file so \fBbtrfs scrub resume\fP can
continue from the last position.
.UNINDENT
.INDENT 0.0
.TP
.B limit [options] <path>
Show or set scrub limits on devices of the given filesystem.
.sp
\fBOptions\fP
.INDENT 7.0
.TP
.B \-d|\-\-devid DEVID
select the device by DEVID to apply the limit
.TP
.B \-l|\-\-limit SIZE
set the limit of the device to SIZE (size units with suffix),
or 0 to reset to \fIunlimited\fP
.TP
.B \-a|\-\-all
apply the limit to all devices
.UNINDENT
.INDENT 7.0
.TP
.B  \-\-raw
print all numbers raw values in bytes without the \fIB\fP suffix
.TP
.B  \-\-human\-readable
print human friendly numbers, base 1024, this is the default
.TP
.B  \-\-iec
select the 1024 base for the following options, according to
the IEC standard
.TP
.B  \-\-si
select the 1000 base for the following options, according to the SI standard
.TP
.B  \-\-kbytes
show sizes in KiB, or kB with \-\-si
.TP
.B  \-\-mbytes
show sizes in MiB, or MB with \-\-si
.TP
.B  \-\-gbytes
show sizes in GiB, or GB with \-\-si
.TP
.B  \-\-tbytes
show sizes in TiB, or TB with \-\-si
.UNINDENT
.TP
.B resume [\-BdqrR] <path>|<device>
Resume a cancelled or interrupted scrub on the filesystem identified by
\fIpath\fP or on a given \fIdevice\fP\&. The starting point is read from the
status file if it exists.
.sp
This does not start a new scrub if the last scrub finished successfully.
.sp
\fBOptions\fP
.sp
see \fBscrub start\fP\&.
.UNINDENT
.INDENT 0.0
.TP
.B start [\-BdrRf] <path>|<device>
Start a scrub on all devices of the mounted filesystem identified by
\fIpath\fP or on a single \fIdevice\fP\&. If a scrub is already running, the new
one will not start. A device of an unmounted filesystem cannot be
scrubbed this way.
.sp
Without options, scrub is started as a background process. The
automatic repairs of damaged copies are performed by default for block
group profiles with redundancy. No\-repair can be enabled by option \fI\-r\fP\&.
.sp
\fBOptions\fP
.INDENT 7.0
.TP
.B  \-B
do not background and print scrub statistics when finished
.TP
.B  \-d
print separate statistics for each device of the filesystem
(\fI\-B\fP only) at the end
.TP
.B  \-r
run in read\-only mode, do not attempt to correct anything, can
be run on a read\-only filesystem
.TP
.B  \-R
raw print mode, print full data instead of summary
.TP
.B  \-f
force starting new scrub even if a scrub is already running,
this can useful when scrub status file is damaged and reports a
running scrub although it is not, but should not normally be
necessary
.UNINDENT
.sp
\fBDeprecated options\fP
.INDENT 7.0
.TP
.BI \-c \ <ioprio_class>
set IO priority class (see \X'tty: link https://man7.org/linux/man-pages/man1/ionice.1.html'\fI\%ionice(1)\fP\X'tty: link' manual page) if the IO
scheduler configured for the device supports ionice. This is
only supported by BFQ or Kyber but is \fInot\fP supported by
mq\-deadline. Please read the section about
\X'tty: link btrfs-scrub//#scrub-io-limiting'\fI\%IO limiting\fP\X'tty: link'\&.
.TP
.BI \-n \ <ioprio_classdata>
set IO priority classdata (see \X'tty: link https://man7.org/linux/man-pages/man1/ionice.1.html'\fI\%ionice(1)\fP\X'tty: link' manpage)
.TP
.B  \-q
(deprecated) alias for global \fI\-q\fP option
.UNINDENT
.TP
.B status [options] <path>|<device>
Show status of a running scrub for the filesystem identified by \fIpath\fP
or for the specified \fIdevice\fP\&.
.sp
If no scrub is running, show statistics of the last finished or
cancelled scrub for that filesystem or device.
.sp
\fBOptions\fP
.INDENT 7.0
.TP
.B  \-d
print separate statistics for each device of the filesystem
.TP
.B  \-R
print all raw statistics without postprocessing as returned by
the status ioctl
.TP
.B  \-\-raw
print all numbers raw values in bytes without the \fIB\fP suffix
.TP
.B  \-\-human\-readable
print human friendly numbers, base 1024, this is the default
.TP
.B  \-\-iec
select the 1024 base for the following options, according to
the IEC standard
.TP
.B  \-\-si
select the 1000 base for the following options, according to the SI standard
.TP
.B  \-\-kbytes
show sizes in KiB, or kB with \-\-si
.TP
.B  \-\-mbytes
show sizes in MiB, or MB with \-\-si
.TP
.B  \-\-gbytes
show sizes in GiB, or GB with \-\-si
.TP
.B  \-\-tbytes
show sizes in TiB, or TB with \-\-si
.UNINDENT
.sp
A status on a filesystem without any error looks like the following:
.INDENT 7.0
.INDENT 3.5
.sp
.EX
# btrfs scrub start /
# btrfs scrub status /
UUID:             76fac721\-2294\-4f89\-a1af\-620cde7a1980
Scrub started:    Wed Apr 10 12:34:56 2023
Status:           running
Duration:         0:00:05
Time left:        0:00:05
ETA:              Wed Apr 10 12:35:01 2023
Total to scrub:   28.32GiB
Bytes scrubbed:   13.76GiB  (48.59%)
Rate:             2.75GiB/s
Error summary:    no errors found
.EE
.UNINDENT
.UNINDENT
.sp
With some errors found:
.INDENT 7.0
.INDENT 3.5
.sp
.EX
Error summary:    csum=72
  Corrected:      2
  Uncorrectable:  72
  Unverified:     0
.EE
.UNINDENT
.UNINDENT
.INDENT 7.0
.IP \(bu 2
\fICorrected\fP \-\- number of bad blocks that were repaired from another copy
.IP \(bu 2
\fIUncorrectable\fP \-\- errors detected at read time but not possible to repair from other copy
.IP \(bu 2
\fIUnverified\fP \-\- transient errors, first read failed but a retry
succeeded, may be affected by lower layers that group or split IO requests
.IP \(bu 2
\fIError summary\fP \-\- followed by a more detailed list of errors found
.INDENT 2.0
.IP \(bu 2
\fIcsum\fP \-\- checksum mismatch
.IP \(bu 2
\fIsuper\fP \-\- super block errors, unless the error is fixed
immediately, the next commit will overwrite superblock
.IP \(bu 2
\fIverify\fP \-\- metadata block header errors
.IP \(bu 2
\fIread\fP \-\- blocks can\(aqt be read due to IO errors
.UNINDENT
.UNINDENT
.sp
It\(aqs possible to set a per\-device limit via file
\fBsysfs/fs/btrfs/FSID/devinfo/scrub_speed_max\fP\&. In that case
the limit is printed on the \fIRate:\fP line if option \fI\-d\fP is specified,
or without it on a single\-device filesystem.  Read more about tat in
section about \X'tty: link btrfs-scrub//#scrub-io-limiting'\fI\%scrub IO limiting\fP\X'tty: link'\&.
.INDENT 7.0
.INDENT 3.5
.sp
.EX
Rate:             989.0MiB/s (limit 1.0G/s)
.EE
.UNINDENT
.UNINDENT
.sp
On a multi\-device filesystem with at least one device limit the
overall stats cannot print the limit without \fI\-d\fP so there\(aqs a not that
some limits are set:
.INDENT 7.0
.INDENT 3.5
.sp
.EX
Rate:             36.37MiB/s (some device limits set)
.EE
.UNINDENT
.UNINDENT
.UNINDENT
.SH EXIT STATUS
.sp
\fBbtrfs scrub\fP returns a zero exit status if it succeeds. Non zero is
returned in case of failure:
.INDENT 0.0
.TP
.B 1
scrub couldn\(aqt be performed
.TP
.B 2
there is nothing to resume
.TP
.B 3
scrub found uncorrectable errors
.UNINDENT
.SH AVAILABILITY
.sp
\fBbtrfs\fP is part of btrfs\-progs.  Please refer to the documentation at
\X'tty: link https://btrfs.readthedocs.io'\fI\%https://btrfs.readthedocs.io\fP\X'tty: link'\&.
.SH SEE ALSO
.sp
\fI\%mkfs.btrfs(8)\fP
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